1. Field of the Invention
The invention relates generally to security devices and more particularly to devices that respond to the capacitance of the human body to give off an alarm when a human approaches a door or grips the doorknob thereof.
2. Description of the Prior Art
Devices of this type are fairly well known. For example, an alarm system that may be hung from a doorknob is disclosed in U.S. Pat. No. 3,623,063. The compact device disclosed therein contains an oscillator whose frequency is altered when someone's hand comes in contact with the doorknob. A tuned circuit remote from the oscillator is used as a filter. When the frequency of the oscillator changes in response to the presence of the hand, the amount of signal passing through the filter is altered (since the filter is frequency sensitive), and this alteration in signal level triggers an alarm mechanism.
Devices of the type disclosed in this patent are satisfactory for use as door alarms, but they must be equipped with some form of sensitivity control or adjustment that must be set manually to compensate for physical characteristics of the door, the temperature, variable components in the circuit and other such parameters. Accordingly, it can be difficult for one who is not technically inclined to use such a device.
Another form of door alarm is disclosed in U.S. Pat. Nos. 3,465,325, 3,648,076 and 4,011,554. These patents all disclose devices that sense 60 cycle signals flowing from the hand of one who grasps a doorknob or other apparatus. Like the device disclosed in U.S. Pat. No. 3,623,063, these 60 cycle sensing devices also require the use of sensitivity controls, as is apparent in U.S. Pat. Nos. 3,465,325 and 4,011,554.
The device disclosed in U.S. Pat. No. 3,648,076 has no sensitivity control, but it may only be installed with one side of its circuit connected to the AC line away from ground. Accordingly, it does not lend itself to easy installation. Such AC powered circuits are also not as portable as a device powered by a battery.
A third type of device is disclosed in U.S. Pat. No. 3,508,239. This device utilizes an oscillator and a tuned circuit, but once again the circuit parameters are critical because of the use of the tuned circuit elements and must be set up with precision if they are to function properly.
A more flexible device is disclosed in U.S. Pat. No. 3,973,208. That patent discloses a very sensitive system in which the transmission of a signal from an oscillator to a receptive amplifier is balanced out in a bridge-like manner. The capacitance of a human hand throws off the delicate balance and creates error signals that are detected by detector amplifiers. However, the circuitry utilized in this device is extremely complex as is apparent from the use therein of four (4) high gain operational amplifiers and a number of transistors used in a complicated circuit arrangement. In spite of its complexity, this circuit requires both a null adjustment and a sensitivity adjustment, and these controls may be difficult for those not mechanically inclined to use such controls.
As will be described in greater detail hereinafter, the doorknob alarm device of the present invention differs from the previously proposed devices by providing an alarm device which may be hung from a doorknob or otherwise used in connection with some metallic device and which does not require initializing or compensating adjustment whatsoever. Also, such alarm device contains no critical circuit parameters within it that can come out of adjustment as the device ages or as the ambient temperature changes. Such a simple alarm device may be used by someone having little or no mechanical skill, and still provides a device which is highly sensitive and does not require any adjustment.
Moreover, the alarm device of the present invention is compact and battery powered, and gives a quick and sure response to the presence of an intruder that is signalled by the hand of the intruder touching a doorknob.